Light, compact electric and electronic devices such as cellular phones, notebook computers, and camcorders have recently been actively developed and produced. Such portable electric and electronic devices operate on battery packs when separate power sources are unavailable. Each battery pack includes at least one battery for driving the portable electric or electronic device.
For economic reasons, the battery pack generally comprises a secondary battery, which can be charged and discharged. Exemplary secondary batteries include nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni-MH) batteries, lithium (Li) batteries and lithium ion (Li-ion) batteries.
Lithium secondary batteries operate at 3.6 V, a voltage three times that of nickel-cadmium batteries or nickel-hydrogen batteries which are widely used as power sources for portable electronic devices. Lithium secondary batteries also have high energy density per unit weight.
Lithium secondary batteries primarily use lithium oxides as positive electrode active materials and carbon materials as negative electrode active materials. Generally, lithium secondary batteries are classified according to the electrolyte used, namely, lithium ion batteries use liquid electrolytes and lithium polymer batteries use high molecular weight electrolytes. Lithium secondary batteries can take various shapes, including cylinders, squares, and pouches.
One typical lithium secondary battery comprises an electrode assembly, which includes a positive electrode plate on which a positive electrode active material is coated, a negative electrode plate on which a negative electrode active material is coated, and a separator positioned between the positive electrode plate and the negative electrode plate. The separator prevents short circuits and allows only lithium ions to pass. The lithium secondary battery further comprises a battery case for containing the electrode assembly, and an electrolyte for enabling movement of lithium ions, which electrolyte is injected into the battery case.
A positive electrode tab is connected to the positive electrode plate, which is coated with a positive electrode active material. A negative electrode tab is connected to the negative electrode plate, which is coated with a negative electrode active material. The electrode assembly is manufactured by laminating the positive electrode plate with the positive electrode tab attached, the negative electrode plate with the negative electrode tab attached, and the separator. After lamination, the positive and negative electrode plates and the separator are wound to form the electrode assembly.
Thereafter, the electrode assembly is contained in the battery case, the electrolyte is injected into the battery case, and the battery case is sealed to complete the lithium secondary battery.
According to another lithium secondary battery, the positive electrode plate, the negative electrode plate, and the separator are wound using a core or mandrel. However, during submersion of the electrode assembly in the electrolyte, the volume changes and expands due to heat generated during charging or discharging. Use of a core or mandrel can increase the volume expansion.
In addition, pressure is applied to the electrode assembly due to gas pressure generated during charging or discharging. Further pressure is applied to the electrode assembly as lithium ions gradually attach to the electrode plate, expanding the electrode plate. This pressure causes the electrode assembly to deform.
Further, the electrode assembly is restricted to the volume of the battery case. Therefore, when the electrode assembly deforms, it can be damaged.
In addition to deformation, the battery may swell, i.e. the battery case may become thicker. When this occurs, the battery does not perform well and cannot be easily mounted on the desired electric or electronic device.